The invention relates to devices used to facilitate fire-resistant, pressure-tight penetrations of decks, bulkheads, walls, floors, and similar partitions. The invention will be described primarily in terms of electrical power, control and communications cables although it will be understood that the same apparatus will also have application to other penetrating items.
Such devices are described in the Brattberg U.S. Pat. Nos. 2,732,226; and 3,282,544 and other patents, and are generally known as through-penetration firestop devices or transits.
Typically these devices consist of a series of rubber modules designed to fit closely around individual cables. The modules are retained in a frame which is attached to the deck, bulkhead, wall, floor, or similar partition. The frame is most commonly steel, but may be made of other materials. Each rubber module is typically molded in two halves, forming a block with a cylindrical center passage. The modules are molded in various sizes to cover the range of cable diameters. The rubber is typically controlled to a durometer hardness range of 65 to 80, to ensure mechanical stability when subjected to high pressures. Accessory items are furnished to anchor rows of modules in place, and to close the final opening in the frame while putting the modules in compression.
Transit systems are used whenever there is a fairly large cable population to be accommodated in limited space. A pressure tight, fire-resistant seal is required, for example, in ships bulkheads, to permit electrical cables to enter water-tight compartments. The effectiveness of the seal depends on a close fit between the individual modules and the cables they enclose, and on the compression and retention of the assembly of modules in the supporting frame.
There are several factors that influence the quality of the fit between the inner groove of the module and the cable. Cables usually are not perfectly round or straight, and their diameters are neither uniform nor exact. Since modules must be molded in discrete sizes, a perfect fit is improbable. In the past, cable jackets have been relatively soft and compliant. The rubber module and cable jacket have been capable of mutual deformation, so that a close fit develops when the system is put in compression.
Improvements in cable jacket and insulation materials in recent years have concentrated on fire performance. New materials have been introduced with low flame spread and smoke emission characteristics. Cable jackets made with these materials are significantly harder and more rigid. The stiffness of the jacket materials has made it more difficult to obtain a close fit of module to the cable. While it might appear that reducing the hardness of the module would produce a compensating ability to conform to cable variations, this approach has proved impractical. When the modules are made sufficiently soft, the column strength of the rubber mass under compression is too low for stability, resulting in excessive deformation of the modules and leakage at lower pressures.
One solution to this problem is to provide modules with interior grooves or contours, creating multiple diameters. The effect of this design is to reduce the bearing surface of the module on cable, increasing the force available to produce mutual deformation. In this way it is possible to maintain good pressure tightness, but at the price of much higher local stress concentration on the cable jacket. The long term effect on cable jacket performance is not known, but believed to be adverse.
The prior art includes the structures described in the following U.S. Pat. Nos. 1,603,991 V. I. Smithers; 1,947,481 F. C. Meyer; 2,732,226; 3,569,608 Louis Ace; 3,655,907 R. A. Philibert et al; Des. 249,558 M. Blomqvist; Des. 249,559 M. Blomqvist 4,291,195 Blomqvist; Des. 253,554 M. Blomqvist; 4,656,313 Moore et al; Des. 714,860 M. Blomqvist; 4,677,253 M. Blomqvist; 4,733,016 M. Twist et al, 4,767,086 M. Blomqvist. These patents while being generally relevant do not suggest a solution to the problems with which the present invention is concerned.
It is an object of the invention to provide a more close fit in a through-penetration firestop device or transit between the individual modules and the cables they enclose. This object of the invention is to improve the fluid pressure seal to thus reduce passage of noxious or explosive gases into compartments.
This object also contemplates prevention the passage of liquids around cable jackets of unusual or irregular shape or design.
It is an object of the invention to provide apparatus which is inexpensive to manufacture as well as requires a minimum of labor to install.
Still another object of the invention is to combine the advantages of the low durometer rubber to conform to variations in cable jacket shape or dimension with high durometer for mechanical strength and stability.
Yet another object of the invention is to accommodate a wider range of cable diameters than would be possible with ordinary modules of prior design, thus reducing both inventory and tooling requirements.
Still another object of the invention is to eliminate the need for convoluted interior surfaces for sealing that can damage cable jackets. Thus it is an object to reduce cable jacket compression that could otherwise lower the dielectric strength of the cable jacket material.
Another object of the invention is to simplify installation requirements by accommodating non-compliant or irregularly shaped penetrating items.
It is also an object of the invention to provide a structure that when exposed to a fire condition the elastomer expands to close apertures caused by the burning away of cable jackets. This effect is known as "intumescence."